1. Field of the Invention
The invention concerns a printing apparatus according of the type that is equipped to print to flat goods to be printed during the passage of flat goods by a printer unit. The printing apparatus can be used in a mail processing system, in particular in a franking and/or addressing machine. The goods are letters and other mail items or flat articles.
2. Description of the Prior Art
In known franking machines, contact pressure elements are used that press the mail good from below against a transport direction that is situated above, and direct the mail items past the print head for printing. For example, elastically borne counter-pressure rollers; counter-pressure elements made of elastic elements in the form of leaf springs; elastically borne flat ribbon belts are used as counter-pressure elements.
A device for printing to flat goods at a transport module is already known from U.S. Pat. No. 7,810,810 B2, which has a number of contact pressure elements that act on the flat good with a spring force through an opening in a feed table of an apparatus. The contact pressure elements are designed as lowerable elastic elements and anchored in a lower housing shell of the apparatus. For example, an actively driven flat ribbon belt for the transport of the flat item is provided as a transport direction in the upper housing shell of the apparatus. Due to the suspension of every single contact pressure element, the possibility advantageously exists of mixed mail processing of different mail goods of 0.1 to 10 mm and more that can follow closely in series (i.e. with a short interval between the mail goods) and enable a high throughput of the franking machine.
In this contact pressure device, it is disadvantageous that the mail item is pressed from below against the flat transport belt only in a line, or in points. Even with a number of contact pressure elements, a flat contact pressure against the flat transport belt situated above is not achieved; only an approximation of this ideal state is possible. Transport disruptions (such as shocks and delays) that negatively affect the print quality occur upon running into the individual contact pressure elements.
A device to lower, position and raise contact pressure elements of a printing apparatus is known from the German Utility Model DE 20 2010 015 351 U1. This device is arranged in a box-shaped module that can be slid like a drawer into the printing apparatus or can be slid out from the printing apparatus. The contact pressure device is elastically mounted on a base plate in the housing of the box-shaped module. The box-shaped module has two guide holders for guide rods or rails for sliding in the box-shaped module, a rocker to lower the contact pressure elements, and shaped parts that interact with the contour of the guide rods for lowering, positioning and raising the contact pressure elements of the contact pressure device in order to initially move the contact pressure device backwards into the printing apparatus while sliding the box-shaped module into the printing apparatus, and to move the contact pressure device forwards towards a transport direction upon reaching a predetermined position. The contact pressure device has brush-like contact pressure elements.
A contact pressure device with sprung elastic contact pressure elements is likewise known from the German Utility Model DE 20 2011 109 208 U1. This contact pressure device presses the mail item over its entire surface against the overlying flat transport belt with a double-spring brush element. A transport disruption no longer occurs in the printing process due to the many, densely arranged brush elements, and a high print quality is achieved even given letter thicknesses up to 10 mm.
Brush elements of a brush body are mechanically coupled with a spring system that in turn has a number of spring elements. The brush elements themselves are typically flexurally elastic to a limited extent. The brush elements compensate for the thickness difference up to a thickness of the mail goods of 3 mm. The entirety of the brush body is additionally elastically lowered at thicknesses as of 3 mm. The brush body is attached to a base plate that is in turn borne in a spring-biased manner on a floor plate that is elastically connected with a chassis. The spring elements are arranged between the base plate and the floor plate as well as between the base plate and the chassis. When a letter arrives between brush body and transport belt, the brush body as a whole is distanced from the transport belt by the letter thickness minus the brush element curvature. Given letters of approximately the same thickness, the detection of the subsequent letter is certain, even if the preceding letter has not yet left the contact pressure region. However, if a thin letter (1 mm thick) follows a thick letter (10 mm thick), the risk exists that the thin letter will not be detected as long as the thick letter is still located in the contact pressure region. For such cases (mixed mail), it must be ensured that a letter may only arrive in the contact pressure region when the preceding letter has exited said contact pressure region.
Otherwise, given the processing of a thick mail items and thin mail items in immediate succession, a flat contact pressure of the thin mail item is not ensured. This leads to poorer printing results. In order to process the mail items of different thickness with a uniformly good print quality, an interval (for instance in brush body lengths) between the successive mail items is necessary. The design of the brush element therefore requires a minimum gap, i.e. a minimum letter interval of approximately one brush body length between the flat mail items in order to ensure the uniform contact pressure. This means either reducing the letter throughput and increased control cost, or exclusion of mixed mail. Given significantly non-uniform letter contents, a complete compensation of such non-uniform thicknesses does not occur, such that the print quality can suffer.
Given subdivision of the brush body analogously to the roller bearing, the risk in turn exists of transition shocks and slippage, with subsequent consequences for the print quality.
A printing apparatus of modular design is described in addition to the contact pressure described above in DE 20 2010 015 354 U1. In an upper part of the printing apparatus, a transport module is arranged which has an actively driven, revolving transport belt. In the lower part of the printing apparatus, the box-shaped module is provided with a contact pressure device to press against flat articles (letters). During the printing, the flat article is transported clamped between the transport belt and the contact pressure device. The module is slid into the operating mode and can be removed from the printing apparatus in service mode. This arrangement is therefore subsequently designated as a mobile contact pressure module.